Polyether ether ketone is a thermoplastic resin having extremely high heat resistance, and further is a type of a super engineering plastic that is excellent in chemical resistance and flame retardance, and significant mechanical strength and dimension stability. The polymer has bee used in applications for automobile parts taking advantage of these excellent characteristics, and in particular, utilization as materials that substitute for metal engine parts has been known in attempts to improve performances of engine parts and to reduce weight. Moreover, utilization in insulating coating of electric wires, electric/electronic associated parts, lead free solder materials, as well as in production lines of parts in electronic circuit substrates, chemicals, solvents and corrosive gas has been also known.
A variety of methods for production of the polymer have been known, and as a method for industrial production, a method in which a nucleophilic substitution reaction of hydroquinone is allowed with benzophenone having a halogen group such as fluorine at both ends in the presence of a base to permit polymerization is the most common method. In such a method, use of diphenyl sulfone as a polymerization solvent has been well known in order to obtain polyether ether ketone having favorable properties. In this respect, Patent Documents 1 to 4 and the like may be referred to. At present, polyether ether ketone has been commercially available mainly from Victrex plc under a trade name of PEEK; however these commercial products are also produced using diphenyl sulfone as a polymerization solvent according to the disclosure of the aforementioned documents.
However, the commercially available polyether ether ketone has been known to be inferior in mold flow performance. For conveniently producing a molded product having a complicated shape by injection molding, superior mold flow performance, i.e., low melt viscosity is demanded. Lowering of the molecular weight of polyether ether ketone leads to decrease in melt viscosity, but mechanical physical properties of the molded product are deteriorated; therefore, achievement of both superior mold flow performance and mechanical physical properties has been difficult.
On the other hand, Patent Document 6 discloses a method for the production of polyether ether ketone using sulfolane as a polymerization solvent. In this document, the mold flow performance of the polyether ether ketone obtained in this manner is unclear although MI value is disclosed, since the measurement conditions are not sufficient.
Meanwhile, since diphenyl sulfone is insoluble in water and has a high boiling point (379° C.), a separation step of diphenyl sulfone using an organic solvent, and a purification step with water have been necessary in order to separate the polymer from diphenyl sulfone after the polymerization and to remove inorganic salts in the polymer. Particularly, with respect to the separation step with an organic solvent, for example, Patent Document 5 discloses a method of removing inorganic salts and diphenyl sulfone, as a solvent, using acetone (twice), water (three times), acetone/ethanol (twice) serially. Thus, separation of the resin takes long time, and results in increase in production costs since a large amount of organic solvents must be consumed.
In addition, even if such complicated separation steps, and a purification step with water are carried out, it was impossible to avoid contamination of the product resin with as an impurity an alkali metal ion derived from a base used in polymerization reaction. As a consequence, applications have been restricted for use involving electric/electronic associated parts for which highly pure articles are particularly demanded.
Patent Document 7 discloses a method for the purification using liquid water with high pressure (for example, 6.0×106 Pa) and high temperature (for example, 260° C.). According to this method, the amount of impurities including an alkali metal ion in the obtained resin is reduced; however, a special pressure resistant apparatus is required for practice of this method, leading to necessity of large facilities, which accounts for increase in production costs.    Patent Document 1: U.S. Pat. No. 4,176,222    Patent Document 2: U.S. Pat. No. 4,320,224    Patent Document 3: U.S. Pat. No. 4,711,945    Patent Document 4: U.S. Pat. No. 5,116,933    Patent Document 5: JP-A No. S59-93724    Patent Document 6: Chinese Patent Application Publication No. 1,817,927    Patent Document 7: JP-T No. 2004-526859 (Japanese Translation of PCT International Publication)